Methods for taking measurements of unidirectional transmission properties, such as packet delay, delay-time fluctuations, or the like, in a telecommunications network, such as the Internet, an intranet, or the like, between at least two measuring computers is available. In such cases, a test packet is transmitted from a first measuring computer to a second measuring computer, the first measuring computer recording the departure time of the outgoing test packet, and transmitting this clock time along with the test packet, and the second measuring computer recording the arrival time of the test packet, and a subtraction operation yielding the difference between the departure time from the first measuring computer and the arrival time in the second measuring computer, to determine the delay time of the test packet, the measuring result. The second measuring computer recognizes the departure time from the first measuring computer, from the test packet which contains the information as a timing mark.
The timing marks can be obtained using various methods: before sending the test packet, the measuring computers determine the clock time from a third computer, via the telecommunications network. The third computer presets the reference time for the two measuring computers.
However, in such cases, time fluctuations arise due to the difference in the transmission times when the clock time is communicated to the measuring computers. This makes the measuring results inaccurate so that they cannot be used when considering the quality of the unidirectional transmission properties in a telecommunications network.
Further, the round-trip measurements in the telecommunications network, i.e., when the packet delay is measured from the first measuring computer via the second measuring computer and back, are also much too imprecise, since a symmetrical connection between the two measuring computers cannot be assumed. For example, the connection from the first measuring computer to the second measuring computer can take a first path, and the connection from the second measuring computer to the first measuring computer, a second path, which is not equal to the first path. In this respect, information regarding the packet delays in the context of these measuring methods also may be unusable when considering the unidirectional transmission properties, e.g., if one were to divide the packet delay of the round-trip measurement by two in order to obtain a unidirectional delay.
However, a more or less guaranteed transmission rate is required when implementing new services in the telecommunications network, e.g., in the Internet, when, for example, sending print jobs to print shops. An upper limit is also required for packet delay and delay-time fluctuation, e.g., for IP telephony and video conferencing.
In this context, the decisive quality feature is the unidirectional packet delay, the delay fluctuations derivable therefrom, the packet losses, the throughput, and the availability.
From this, one then guarantees to the customer, for one or more of these parameters, maximum values for packet delays, delay fluctuations, and losses, and/or minimum values for the throughput. In addition, it must be verifiable that these values are being observed by the service provider and the customer.
In this context, the unidirectional packet delay corresponds to the difference between point in time t1 when the first bit of a test packet was sent by a first measuring computer, and point in time t2, when the last bit of the test packet was received by the second measuring computer. The packet delay Dnetwork in a telecommunications network thus yields Dnetwork=t2−t1.
Unidirectional delay-time fluctuations are understood to be the differences between the various delay times of the test packets from a first measuring computer—source—to the second measuring computer—drain. The delay-time fluctuation is always considered for only one transmission direction.
In defining, the following distinction is also made:
A pair of test packets is transmitted from one defined source, or a first measuring point, to a defined drain, or a second measuring point. The delay-time fluctuation is then the difference between the measured delay time of the second test packet and the measured delay time of the first test packet of the transmitted pair of test packets.
A stream of test packets is transmitted from one defined source, or a first measuring point, to a defined drain, or a second measuring point. A packet stream is formed, in this context, by logically successive test packets,—numbered packets—, which are transmitted in a fixed sequence. Here, the delay-time fluctuation is the difference between the measured unidirectional delay time of a test packet and the measured delay time of the predecessor packet. Generally, it holds that: tjitter=Dn−Dn-1. Dn is the unidirectional delay time of test packet n, and Dn-1 is the delay time of packet n−1. tjitter is then the delay-time fluctuation. The occurrence of the delay-time fluctuation is a direct consequence of different delay times of the test packets.
A packet loss is understood to be when the first bit of an individual test packet, which is sent from a defined source to a defined drain, does not reach the drain. One also speaks of a packet loss when a test packet arrives at the receiver, but has at least one corrupted bit, or when the delay time of a test packet exceeds a predefined time period, such as 255 seconds.
In this context, only one transmission direction is considered. The measuring data are recorded in that a received test packet counts as “1”, a packet loss as “0”. The packet loss is measured over a defined time interval. At the receiver, the delay resulting from the transit time is to be considered in selecting the measuring interval.